The invention relates to an arrangement for power transmission in a combustion engine, of the type which recovers energy from the exhaust gases. The invention also relates to a method for power transmission in a combustion engine, of the type which recovers energy from the exhaust gases.
When designing combustion engines it is for several reasons, for example with regard to the environment and for reasons of cost, important to obtain a high degree of efficiency in the engine. It can be noted that the combustion gases which leave the combustion chamber in the combustion engine contain significant amounts of energy. In order to utilize these amounts of energy which would otherwise be lost, a turbo unit of a conventional kind can be connected downstream of the outlet from the exhaust chamber. Such a turbo unit essentially consists of a turbine which is rotated by the exhaust flow. The energy which is thus absorbed by the turbine is then transferred to a compressor which compresses the air on the intake side of the combustion engine. This means that a larger amount of fuel can be fed to a combustion chamber of a certain given size, whereby the efficiency of the engine can be increased.
The energy in the exhaust gases can also be recovered by leading the exhaust flow past a so-called turbo compound unit. Such a unit normally comprises two turbines which are arranged serially in the exhaust system of the engine, downstream of the combustion chamber. Both of the turbines are powered by the exhaust gases which flow through the exhaust system. The first turbine is part of a turbo unit of the above-mentioned kind, and is connected to a compressor belonging to the turbo unit. The other turbine is connected to the crankshaft of the engine. The energy which is absorbed by the second turbine is in this manner transferred to the crankshaft, thus giving it additional energy.
It is also possible to connect the two units in parallel to each other. It is of course also possible to use a single turbine which drives the crankshaft, without combining it with a conventional turbo unit.
Furthermore, a similar system can be utilized to transfer power from the crankshaft to another engine component, for example, a compressor which can be arranged in connection with the air intake of the engine. In this case, the power from the crankshaft is used to increase the amount of air which is supplied to the engine.
The above-mentioned systems have a common denominator in that they need a power transmission, i.e. a clutch and a gear mechanism, which is arranged between the crankshaft and the turbine (alternatively the compressor). In this way, the revolutions of the crankshaft, which during normal operation is approximately 2000 rpm, can be adjusted to the revolutions of the turbine (alternatively the compressor), which normally is approximately 100000 rpm.
In connection with combustion engines, the crankshaft is affected in a pulse-like manner by power impulses from the different cylinders. This causes variations in the number of revolutions of the crankshaft, which in turn causes noise problems and a risk for increased wear of the turbo compound unit. In particular, the variations in revolution cause an increased risk of high gear-loads in transmissions, and a risk of fatigue in the spindles of the turbines. The amplitude of the variations in revolution is furthermore highly amplified by the gearing in the above-mentioned power transmission. This imparts especially high demands on the power transmission.
Power transmission in a turbo compound unit can, according to previously known technology, be achieved with some kind of a mechanical connection. For example SE 8904374-9 shows the fitting of a cog-wheel on the crankshaft of the combustion engine, and connecting a turbine of a turbo compound unit to the cog-wheel via a number of cog-wheels and a hydrodynamic coupling. Such a hydrodynamic coupling consists of two mechanically separate, shovel-like members, which can rotate relative to each other inside a casing. These members have opposite surfaces which are positioned at a certain distance from each other, so that a slit-shaped space is formed between the surfaces. Oil is supplied to the space from a special feeding system. The oil can furthermore be transported to an oil trough and subsequently be re-used. During rotation of the first member, a torque is transferred to the second member, since the mass of the oil which is affected by the first member creates kinetic energy, which in turn transfers a torque to the second member.
A drawback of the hydrodynamic coupling is that the difference in the number of revolutions between the first and the second member is relatively large. This causes a high loss of power in the form of heat, which is transferred to the oil inside the casing. This power-loss must be removed by cooling, which in turn creates a need for special measures in the form of optimization and dimensioning of a cooling system. This is of course a drawback as regards cost and packing.
Another drawback of the hydrodynamical coupling relates to the fact that the rotating members due to centrifugal force eject particles, for example soot, against the inner walls of the casing. These particles are deposited on the inner walls of the casing, and can in the worst case cause clogging of the feeding or exhaust passage for the oil. Due to this, there is a need for expensive measures. Since the depositing of particles obstructs the transport of oil, the oil must be filtered before it can be re-used. This in turn creates a need for some kind of oil-filter, which must be used in order to have as few particles as possible fed into the casing. Alternatively, the inner walls of the casing can be provided with some kind of coating, for example teflon, in order to reject particles. This is also a drawback, since it increases the costs.
A further drawback when using hydrodynamic couplings is that they need a relatively large flow of oil in order to function. This creates high demands for the oil-supply system in the vehicle in which the turbo compound unit is used.
Another way of attenuating those vibrations which are caused in a turbo compound unit during operation of the engine is to use some kind of special attenuating device. For example SE 7807407-7 shows the use of a vibration attenuating device. Naturally, advantages as regards packing and cost would be obtained if there were not a need for a special vibration attenuating device when transferring power in the turbo compound unit.
The object of the present invention is to obtain an improved arrangement for recovering energy from the exhaust gases of a combustion engine, in which the drawbacks of previously known arrangements are eliminated. This object is achieved by means of an apparatus and a method as disclosed herein.
The invention comprises at least one device for absorbing energy from engine exhausts, and at least one device for compressing air to the engine. The invention furthermore comprises a power transmission between at least one of said devices and a crankshaft of the engine. The power transmission comprises power transmission means for transmitting power via shear forces in a viscous medium. In this way, the vibrations which occur during operation of the engine can be attenuated at the source of the disturbance, before they are amplified by gear-shifting. For example, torsional vibration in the crankshaft can be attenuated in an efficient manner by means of the invention. Furthermore, a lower noise level is obtained when engine braking, as are reduced cog-loads and a reduced risk for fatigue of rotor spindles. Furthermore, no extra filtering of engine oil is necessary, which is the case when using a hydrodynamic coupling. In comparison to the previously known hydrodynamic coupling, a better degree of efficiency for a turbo compound unit is also obtained with the invention.
Advantageous embodiments of the invention will become evident from the dependent claims.